arXiv subjects

Sarif Khan

Publications and source records attributed to Sarif Khan.

At least 19 recordsLinked to original sources

arXiv2026

Pseudo-scalar dark matter from a broken gauged symmetry

We propose a novel model for pseudo-scalar dark matter (PSDM) by extending the Standard Model (SM) with a dark gauged $U(1)_X$ symmetry, but without dark charged fermions. We impose a $Z_2$ symmetry to ensure the stability of pseudo-scalar dark matter and regard the $U(1)_X$ symmetry as being broken dominantly by a large VEV of the singlet scalar field. The would-be Goldstone associated with the $U(1)_X$ gauge boson is almost orthogonal to the direction of PSDM. As a result, we show that PSDM appears as a stable pseudo-Nambu-Goldstone boson receiving the mass from the $U(1)_X$ invariant mixing potential and the corresponding cross section for direct detection gets suppressed even for the weak-scale mass of PSDM. We also show that the correct relic density can be explained by the PSDM annihilations into the SM particles or into a pair of light Higgs-like scalars, being compatible with the bounds from Higgs invisible decay, Higgs data and indirect detection.

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arXiv2026

Baryogenesis and Dark Matter from non-thermally produced WIMPs

We illustrate, via a simplified model, a scenario in which the baryon-asymmetry and, possibly the dark matter component of the Universe are simultaneously generated by the decay of a WIMP-like mother particle, in turn produced non-thermally during an epoch of Early Matter domination. We first consider the standard evolution of the Universe and introduce TeV-scale BSM particles, finding that this paradigm cannot produce enough baryon asymmetry. This deficiency can be resolved by considering a non-standard scenario, with a matter-dominated phase prior to radiation-domination. Finally, we include a dark matter candidate, which is non-thermally produced during the Early Matter domination. Our results demonstrate an interesting common origin of baryon asymmetry and Dark Matter, with the particle masses lying within the collider-detectable range, thanks to the presence of non-standard evolution in the early Universe.

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arXiv2025

Low-reheating scenario in dark Higgs inflation and its impact on dark photon dark matter production

We investigate dark matter (DM) phenomenology and cosmic inflation within a unified framework based on a dark $U(1)_D$ gauge extension of the Standard Model (SM). The associated dark gauge boson, namely the dark photon, serves as a viable DM candidate, which we call dark photon dark matter (DPDM), whilst the dark Higgs field drives inflation. We explore a low-reheating scenario where DM production occurs during reheating, resulting in significant entropy dilution of the DPDM abundance. Both weakly interacting massive particle (WIMP) and feebly interacting massive particle (FIMP) DM scenarios are explored, depending on the dark gauge coupling strength. For FIMP-type DM, the entropy dilution allows for stronger couplings whilst maintaining the correct relic abundance, potentially bringing these candidates within the reach of current and near-future detection experiments. Similarly, WIMP-type DM can be realised with weaker couplings. We perform a comprehensive parameter scan incorporating constraints from collider data, DM direct and indirect detection experiments, and cosmological observations. Taking quantum corrections and running of the couplings into account, we demonstrate that dark Higgs inflation yields predictions for the spectral index $n_s$ and the tensor-to-scalar ratio $r$ that are consistent with the Planck, BICEP/Keck, and ACT data. The nonminimal coupling of the dark Higgs inflaton field to gravity is shown to be much smaller than in the case of the SM Higgs inflation scenario, avoiding unitarity concerns. We show that reheating temperatures as low as 1 GeV and 1 MeV can be achieved through the decay and scattering processes of the inflaton, respectively, with the latter allowing for larger Higgs mixing angles and enhanced detection prospects. Our results establish that this minimal extension successfully unifies DM physics with inflationary cosmology.

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arXiv2025

Decaying vector dark matter with low reheating temperature for KM3NeT signal and its impact on gravitational waves

We propose a new model to explain the KM3NeT neutrino event through a low reheating scenario with a suppression in the GW spectrum originating from cosmic string networks. To achieve this, we extend the SM gauge sector by an abelian gauge symmetry and a singlet scalar. Once the abelian gauge symmetry spontaneously breaks, the extra gauge boson acquires mass and becomes a suitable Dark Matter (DM) candidate. Due to the kinetic mixing with the hypercharge gauge group, DM can decay into SM particles. To explain the KM3NeT signal, we need $\mathcal{O}(100)$ PeV DM, which can be produced in the correct order of DM density in a low reheating scenario. In this scenario, the overabundance issue of heavy DM can be tackled by diluting its abundance through the continuous injection of entropy when the matter-like inflaton decays into the SM bath. Using the low reheating scenario, we can obtain the correct value of DM density both for freeze-out and freeze-in mechanisms for super-heavy DM. Moreover, we have studied the Gravitational Waves (GWs) produced from cosmic strings, which fall within the detectable range of future proposed GW experiments. Additionally, the dominance of a quadratic inflaton potential before the reheating temperature changes the temperature-scale factor relation, which suppresses the GW spectrum at higher frequencies. Choosing an arbitrarily low reheating temperature provides only a tiny fraction of the DM density due to dilution from entropy injection. This fraction of the vector DM suggests that only the extragalactic contribution is relevant in the KM3NeT event because DM lifetime is shorter than the age of the Universe.

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arXiv2025

Constraining Inflation via FIMP dark matter using the $\beta$-function with collider implications

The present study connects inflation and freeze-in type dark matter (DM) within the same setup. Although the observables in these two phenomena lie at vastly different energy scales, they have been properly handled using the RG running of couplings. For studying DM and inflation, the SM has been minimally extended by introducing an abelian dark gauge symmetry and a dark singlet scalar. In studying inflation, the SM Higgs doublet has been considered as the inflaton, which has a non-minimal coupling with the Ricci scalar. All inflationary observables have been computed at the horizon exit scale and constrained using the Planck data. Moreover, inflationary constraints have revealed strong correlations among model parameters, significantly reducing the allowed parameter space. In particular, in the Higgs mixing angle and BSM Higgs mass plane, only those values that ensure the Higgs quartic coupling remains above 0.18 are allowed. The additional gauge boson serves as a suitable DM candidate, produced via the freeze-in mechanism and stabilised by charge conjugation symmetry. The upper bound on the DM relic density further shrinks the parameter space allowed from inflationary constraints, becoming even narrower if we assume that the present vector DM constitutes the total DM density. Since DM interactions are feeble, it remains safe from all terrestrial experimental constraints. Additionally, the feeble dark matter coupling requires the dark Higgs-Ricci scalar non-minimal coupling to be negligible to satisfy Higgs inflation conditions. Finally, we have explored collider aspects and found that the trilinear and quartic Higgs vertices deviate from their SM values after incorporating inflation and DM constraints. Therefore, once we measure $\kappa_{3,4}$ at the future collider, we can establish the robustness of the Higgs inflation scenario.

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arXiv2025

Evading Dark Matter Bounds through NLSP-Assisted Freeze-Out with Long-Lived Signatures

In this work, we explore a conversion-driven freeze-out scenario, where the next-to-lightest stable particle (NLSP) sets the dark matter (DM) abundance through the process ``NLSP SM $\leftrightarrow$ DM SM". Although DM is produced via a freeze-out mechanism, its interaction strength with the visible sector can range from weak to feeble couplings. This results in a vast, largely unexplored parameter space that evades current direct, indirect, and collider bounds, while remaining testable in the near future. We study this mechanism in the context of an alternative $U(1)_{B-L}$ model, where four chiral fermions are required to cancel gauge anomalies, unlike the usual case with three right-handed neutrinos. The observed relic abundance is successfully reproduced within this framework. The viable parameter space can be probed by future direct detection experiments, while remaining inaccessible to indirect searches. Our results show that the DM relic density is highly sensitive to the NLSP-SM interaction strength and the mass difference between the NLSP and DM, but not to the DM-SM direct interaction. For certain parameter choices, the NLSP decays to DM via two or three body processes involving an extra gauge boson and SM particles, leading to long-lived decays outside the CMS or ATLAS detectors at the LHC. In contrast, if the decay proceeds via a CP-odd Higgs, it occurs promptly within the detector. We investigate prospects for detecting such long-lived NLSPs at the proposed MATHUSLA detector, with similar expectations for the ongoing FASER experiment. Finally, we find that choosing arbitrarily small values of the gauge coupling or BSM fermionic mixing angle can violate successful BBN predictions.

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arXiv2025

Linking the KM3-230213A Neutrino Event to Dark Matter Decay and Gravitational Wave signals

The KM3NeT collaboration recently reported the detection of an ultra-high-energy (UHE) neutrino event, dubbed KM3-230213A. This is the first observed neutrino event with energy of the order of $\mathcal{O}(100) {\rm PeV}$, the origin of which remains unclear. In this paper, we interpret this high energy neutrino event in terms of the Dirac fermion dark matter (DM) $\chi$ decays via the right-handed (RH) neutrino portal assuming the Type-I seesaw mechanism for neutrino masses and mixings. Furthermore, the Dirac fermion dark matter $\chi$ is assumed to be charged under $U(1)_X$ dark gauge symmetry, which is spontaneously broken by the vacuum expectation value (VEV) of the dark Higgs $\Phi$. In this scenario, DM can decay into a pair of Standard Model (SM) particles, such as neutrinos, leptons, and gauge bosons via the RH neutrino portals for $v_\Phi \gg m_\chi$. Then we can reply on the HDMSpectra package to generate the neutrino and $\gamma$-ray spectra from heavy DM decays. If the DM mass is around $440\ {\rm PeV}$ with a lifetime $5\times 10^{29}$ sec, it can account for the KM3-230213A event. However, such heavy DM cannot be produced through the thermal freeze-out mechanism due to overproduction and violation of unitarity bounds. We focus on the UV freeze-in production of DM through a dimension-5 operator, which helps in producing the DM dominantly in the early Universe. Finally, the large value of the dark Higgs field VEV opens up the intriguing possibility of generating gravitational waves (GWs) spectra from cosmic strings. We have found a reasonable set of parameter values that can address the KM3NeT signal, yield the correct value of the DM relic density through freeze-in mechanism, and allow for the possible detection of GW signal at the future detectors.

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arXiv2025

Multicomponent Dark Matter with Collider Implications

The present work aims to study an extension of the Standard Model (SM) that addresses the prominent SM shortcomings, i.e., can explain the neutrino mass, the dark matter (DM) content, and the matter-antimatter asymmetry of the Universe. The model introduces the possibility of a multicomponent DM scenario leading to distinctive signals at colliders. The SM is extended by a ``dark" $SU(2)_{D}$ gauge symmetry and new fermions and scalar doublets, charged only under $SU(2)_{D}$, that provide candidates for a multicomponent DM. Previously, we have considered in this model the asymmetric DM scenario, while in the present work, we explore the symmetric DM case. We focus on the parameter region where the dark fermion DM annihilates dominantly into the additional dark gauge bosons and the ``inert" doublet DM annihilates to SM states via the SM Higgs resonance. This particular choice of doublet mass ensures that the heavier BSM Higgs always has one decay mode open to DM leading to the possibility of detecting such particles at the LHC, in the missing energy plus dijet ($\cancel{E}_{T}+2j$) final states. We also discuss the prospects for detecting DM through direct and indirect detection experiments and via Long-Lived-Particle searches. Finally, we show that, as typical of other WIMP models, for low DM mass, signals can be expected in future collider experiments, but for the higher mass range above $500$ GeV we have to rely solely on direct detection experiments. Both types of experiments will be essential to fully cover the allowed parameter space.

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arXiv2025

Higgs portal vector dark matter at a low reheating temperature

In this study, we explore vector dark matter (DM) production in the early Universe focusing on a scenario with a low reheating temperature. One can achieve low reheat temperature in many ways, for example, by considering a longer lifetime of the inflaton field. We analyze the impact of various model parameters on DM production, including gauge coupling and reheat temperature, while incorporating all relevant constraints from DM relic density, collider bounds, and DM direct and indirect detection experiments. Our results reveal a strong correlation between DM mass and reheat temperature, with viable parameter space requiring $T_R/M_{W_D} \sim 0.1$. While DM production from decays is generally subdominant, we identify a regime where freeze-in production from decay is dominant due to the phase space suppression. For DM masses below 100 GeV, production is primarily driven by SM fermions, whereas higher masses come due to the Higgses annihilation. The enhanced coupling strength in our framework enables potential detection in direct and indirect detection and collider experiments. The direct detection experiments have already explored some parts of the region and future DARWIN will explore the further region whereas for indirect detection, the detection prospects for the present case are futile. We found that a very narrow region of the parameter space has been explored by the DM direct detection contrary to the WIMP DM case where most of the parameter space has been ruled out.

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arXiv2025

WIMP-FIMP option and neutrino masses via a novel anomaly-free B-L symmetry

We propose a novel $U(1)_{B-L}$ model with singlet dark matter fermions composed of WIMP and FIMP, which is anomaly-free without a need for introducing right-handed neutrinos. Fermion dark matter masses are generated after the $U(1)_{B-L}$ is broken spontaneously, so the Yukawa couplings for WIMP and FIMP components can be distinguished by the hierarchical values of the vacuum expectation values of the single scalar fields. Moreover, the $U(1)_{B-L}$ gauge boson receives a TeV-scale mass for a tiny extra gauge coupling, so it goes out of equilibrium from the rest of the model content in the early Universe. Both the $U(1)_{B-L}$ gauge boson and FIMP component are produced from the decays of the bath particles, and the former can decay into FIMP DM and/or WIMP DM before BBN. The WIMP component can reside in the resonance region of the Higgs bosons or dominantly annihilate into a pair of singlet-like scalars. Thus, there is a flexibility to choose a small mixing between the visible and dark sectors, thereby evading all the current direct and indirect detection bounds. Furthermore, we show that WIMP and FIMP components can coexist in suitable fractions, depending on the choice of model parameters, allowing for additional protection for WIMP DM against various experimental bounds. Finally, we identify the dimension-6 and dimension-7 operators for Majorana neutrino masses in our model, being consistent with the $U(1)_{B-L}$ gauge symmetry, and provide a possibility of extending the model with additional singlet fermions for neutrino masses.

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arXiv2024

Multi-component dark matter and Galactic 511 keV $\gamma$-ray emission

We study multi-component dark matter scenarios and the Galactic 511 keV $\gamma$-ray emission line signal in the framework of a local, dark $U(1)_D$ extension of the Standard Model. A light vector dark matter particle associated with the dark $U(1)_D$ may decay and annihilate to electron-positron pairs. The produced positrons may in turn form positroniums that subsequently annihilate to two photons, accounting for the observed line signal of the Galactic 511 keV $\gamma$-ray emission. Three scenarios are investigated. First, we consider the minimal $U(1)_D$ extension where a dark gauge boson and a dark Higgs boson are newly introduced to the particle content. As a second scenario, we consider WIMP-type dark matter with the introduction of an extra dark fermion which, in addition to the dark gauge boson, may contribute to the dark matter relic abundance. It is thus a multi-component dark matter scenario with a UV-complete dark $U(1)_D$ symmetry. In particular, the vector dark matter may account for a small fraction of the total dark matter relic abundance. Finally, we consider the scenario where the dark matter particles are of the FIMP-type. In this case, both the light vector and fermion dark matter particles may be produced via the freeze-in and super-WIMP mechanisms. Considering theoretical and observational constraints, we explore the allowed parameter space where the Galactic 511 keV $\gamma$-ray line signal and the dark matter relic can both be explained. We also discuss possible observational signatures.

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arXiv2024

Constraining dark matter from strong phase transitions in a $U(1)_{L_μ-L_τ}$ model: Implications for neutrino masses and muon $g-2$

In this paper, we study a non-minimal gauged $U(1)_{L_μ-L_τ}$ model, where we add two complex singlet scalars, three right-handed Majorana neutrinos (RHN), and a vector-like dark fermion to the Standard Model (SM), all non-trivially charged under the extra gauge symmetry. The model offers an easy resolution to the muon $(g-2)$ anomaly, which fixes the scale of spontaneous symmetry breaking. Furthermore, the two-zero minor structure in the RHN mass matrix provides successful predictions for neutrino oscillation parameters, including the Dirac phase. The extended scalar sector can easily induce first-order phase transitions. We identify all possible phase transition patterns in the three-dimensional field space. We quantify the associated gravitational waves from the sound wave source and demonstrate that the signatures can be observed in future space-based experiments. We find that strong first-order phase transitions require large values of scalar quartic couplings which constrain the scalar dark matter (DM) relic density to a maximum of $10^{-2}$ and $10^{-5}$ when we consider the DM direct detection bound. Nonetheless, the model successfully explains the DM relic density via contribution from the vector-like dark fermion. We show the allowed range of the model parameters that can address all the beyond SM issues targeted in this study.

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arXiv2024

Interplay between Higgs inflation and dark matter models with dark $U(1)$ gauge symmetry

We investigate dark matter phenomenology and Higgs inflation in a dark $U(1)_D$-extended model. The model features two dark matter candidates, a dark fermion and a dark vector boson. When the fermion dark matter $ψ$ is heavier than the vector dark matter $W_D$, there is an ample parameter space where $ψ$ is dominant over $W_D$. The model can then easily evade the stringent bounds from direct detection experiments, since $ψ$ has no direct coupling to the Standard Model particles. Furthermore, the model can accommodate inflation in three different ways, one along the Standard Model Higgs direction, one along the dark Higgs direction, and one along the combination of the two. Considering the running of the parameters and various observational constraints, we perform a detailed numerical analysis and identify allowed parameter spaces that explain both dark matter and Higgs inflation in a unified manner. We discuss in detail how the imposition of Higgs inflation severely constrains the dark matter parameter space. The existence of the dark Higgs field is found to play a crucial role both in dark matter phenomenology and in generalised Higgs inflation.

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arXiv2023

Axion Dark Matter and additional BSM aspects in an extended 2HDM setup

We illustrate and discuss the phenomenology of a model featuring a two-Higgs doublet sector augmented by two $SU(2)$ singlet scalars. The gauge symmetry group is extended as well with a $U(1)_{B_{i}-L_{i}}$ component whose spontaneous breaking leads to the gauge boson which has an important effect in the muon (g-2). A global PQ symmetry is introduced upon its breaking we have the axion particle which is also DM in our work. In particular, we have focussed on Type-X and Type-II 2HDM models and found out that (g-2) can not be explained only by the scalar sector for Type-II 2HDM mainly due to stringent constraint from $b \rightarrow s γ$ resulted in $M_{H^{\pm}} > 800$ GeV. For Type-II 2HDM, we can have axion coupling with the gluons which generates the axion potential and possible explanation for the strong CP problem. The proposed model accommodates neutrino masses via the Type-I see-saw mechanism with an upper bound on the right-handed neutrino mass 1 GeV (1 TeV) for Type-II (Type-X) 2HDM due to the presence of Planck scale suppressed operators. Moreover, we also have additional scalars which affect the oblique parameters and hence the W-boson mass which leads us to explain the W-boson mass observed at CDF-II detector. The most stringent constraints on the masses and quartic couplings come from the perturbativity and potential bound from below conditions which leads to fine-tuning among the parameters in part of the parameter space. Finally, we discuss the possible detection prospects of the axion DM and the additional gauge boson.

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arXiv2023

Reviewing the prospect of fermion triplets as dark matter and source of baryon asymmetry in non-standard cosmology

Indirect searches of Dark Matter (DM), in conjugation with `missing track searches' at the collider seem to confine SU(2)$_L$ fermion triplet DM (FTDM) mass within a narrow range around 1 TeV. The canonical picture of the pure FTDM is in tension since it is under-abundant for the said mass range. Several preceding studies have reported that an extra species ($ϕ$), redshifts faster than the radiation ($\sim a^{-(4+n)}$ where $n>0$), leads to a faster expanding early Universe by dominating in the energy density with an enhanced Hubble parameter. This has the potential to revive the under-abundant FTDM ($\mathbb{Z}_2$ odd, lightest generation) by causing freeze-out earlier without modifying the interaction strength between DM and thermal bath. On the other hand, although the CP asymmetry produced due to the decay of $\mathbb{Z}_2$ even heavier generations of the triplet remains unaffected, its evolution is greatly affected by the non-standard cosmology. It has been observed through numerical estimations that the minimum mass of the triplet, required to produce sufficient baryon asymmetry of the Universe (BAU), can be lowered up to two orders (compared to the standard cosmology) in this fast expansion scenario. The non-standard parameters $n$ and $T_r$ (a reference temperature below which radiation dominance prevails), which simultaneously control DM abundance as well as the frozen value of BAU, are tightly constrained from the observed experimental values. We have found that $n$ is strictly bounded within the interval $0.4\lesssim n \lesssim 1.8$ where the upper bound is imposed by the BAU constraint whereas the lower bound arises to satisfy the correct DM abundance. It has been noticed that the restriction on $T_r$ is not so stringent as it can vary from sub-GeV to a few tens of GeV.

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arXiv2022

A two-component vector WIMP -- fermion FIMP dark matter model with an extended seesaw mechanism

We consider an extension of the Standard Model that explains the neutrino masses and has a rich dark matter phenomenology. The model has two dark matter candidates, a vector WIMP and a fermion FIMP, and the sum of their relic densities matches the total dark matter abundance. We extensively study the dark matter production mechanisms and its connection with the neutrino sector, together with various bounds from present and future experiments. The extra scalar field in the model may induce a first-order phase transition in the early Universe. We study the production of stochastic gravitational waves associated with the first-order phase transition. We show that the phase transition can be strong, and thus the model may satisfy one of the necessary conditions for a successful electroweak baryogenesis. Detectability of the phase transition-associated gravitational waves is also discussed.

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arXiv2022

WIMP and FIMP Dark Matter in Singlet-Triplet Fermionic Model

We present an extension of the SM involving three triplet fermions, one triplet scalar and one singlet fermion, which can explain both neutrino masses and dark matter. One triplet of fermions and the singlet are odd under a $Z_2$ symmetry, thus the model features two possible dark matter candidates. The two remaining $Z_2$-even triplet fermions can reproduce the neutrino masses and oscillation parameters consistent with observations. We consider the case where the singlet has feeble couplings while the triplet is weakly interacting and investigate the different possibilities for reproducing the observed dark matter relic density. This includes production of the triplet WIMP from freeze-out and from decay of the singlet as well as freeze-in production of the singlet from decay of particles that belong to the thermal bath or are thermally decoupled. While freeze-in production is usually dominated by decay processes, we also show cases where the annihilation of bath particles give substantial contribution to the final relic density. This occurs when the new scalars are below the TeV scale, thus in the reach of the LHC. The next-to-lightest odd particle can be long-lived and can alter the successful BBN predictions for the abundance of light elements, these constraints are relevant in both the scenarios where the singlet or the triplet are the long-lived particle. In the case where the triplet is the DM, the model is subject to constraints from ongoing direct, indirect and collider experiments. When the singlet is the DM, the triplet which is the next-to-lightest odd particle can be long-lived and can be probed at the proposed MATHUSLA detector. Finally we also address the detection prospects of triplet fermions and scalars at the LHC.

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arXiv2022

$(g-2)_{e,\,μ}$ and strongly interacting dark matter with collider implications

The quest for new physics beyond the Standard Model is boosted by the recently observed deviation in the anomalous magnetic moments of muon and electron from their respective theoretical prediction. In the present work, we have proposed a suitable extension of the minimal $L_μ-L_τ$ model to address these two experimental results as the minimal model is unable to provide any realistic solution. In our model, a new Yukawa interaction involving first generation of leptons, a singlet vector like fermion ($χ^{\pm}$) and a scalar (either an SU(2)$_{L}$ doublet $Φ^\prime_2$ or a complex singlet $Φ^\prime_4$) provides the additional one loop contribution to $a_{e}$ only on top of the usual contribution coming from the $L_μ-L_τ$ gauge boson ($Z_{μτ}$) to both electron and muon. The judicious choice of $L_μ-L_τ$ charges to these new fields results in a strongly interacting scalar dark matter in $\mathcal{O}({\rm MeV})$ range after taking into account the bounds from relic density, unitarity and self interaction. The freeze-out dynamics of dark matter is greatly influenced by $3\rightarrow2$ scatterings while the kinetic equilibrium with the SM bath is ensured by $2\rightarrow2$ scatterings with neutrinos where $Z_{μτ}$ plays a pivotal role. The detection of dark matter is possible directly through scatterings with nuclei mediated by the SM $Z$ bosons. Moreover, our proposed model can also be tested in the upcoming $e^+e^-$ colliders by searching opposite sign di-electron and missing energy signal i.e. $e^{+} e^{-} \rightarrow χ^{+} χ^{-} \rightarrow e^{+} e^{-} \cancel{E}_T$ at the final state.

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